This invention relates to a structure for supporting structures on offshore platforms. More particularly, but not by way of limitation, this invention relates to a structure and method for supporting and compensating for movement during well intervention operations on offshore platforms.
Hydrocarbons are located in subterranean reservoirs. The search for hydrocarbons has brought operators to remote and exotic areas of the globe. Deep water tracts have been explored and drilled with increasing frequency in recent years. Platforms set in waters of 1,000 to 2,000 feet have become common place, and in some instances, wells have been drilled in water depths of over 5,000 feet. Different types of drilling and production platforms have been used in these deep waters. One type of platform is a tension leg platform (TLP). In the TLP, a floating platform is connected to the ocean floor via tendons such as steel cables, as is well understood by those of ordinary skill in the art. Another type of structure used in deep water is the spar platform which generally is a floating cylindrical structure that is anchored to the ocean floor with steel cable means. Other types of floating platforms are known in the art. In deep water, a fixed leg type of platform is generally not an option due to the extreme water depths. The exploration, drilling, completion and production in these deep waters is very expensive. For instance, day rental rates for floatable drilling rigs can cost over $300,000 per day in some instances.
Wave conditions may cause a cyclic buoyant force on these floating platforms based on the raising, lowering, heaving and pitching of the platform. Moreover, tidal conditions may cause a variation in platform height and cause similar buoyant forces. The applied forces will in turn cause motion on the platform and on the work deck of the platform. Additionally, the subterranean well that is drilled will have a riser extending from the sea floor to the platform. In other words, a riser extends from the sea floor to the floating platform. As will be understood by those of ordinary skill in the art, the riser generally does not move in unison with the platform since the riser is fixed to the sea floor by different attachment means. Also, the riser does not experience the same buoyant forces as the floating platform.
While an operator is in the midst of performing well work, the motion of the platform can have detrimental effects on the equipment and ongoing operations. For example, a coiled tubing unit that is rigged-up and running a string of tools into the well could be lifted upward and/or downward, and/or sideways due to the motion of the platform. This motion could potentially cause serious damage such as breaking the connection of the coiled tubing to the riser which in turn could lead to a catastrophic failure. With prior art designs, operators find it necessary to stop operations and rig down the connection and then reconfigure the equipment.
In the course of performing well intervention work on these offshore wells, the operator must install numerous devices such as coiled tubing injector heads, lubricators, blow out preventors, etc. As readily understood by those of ordinary skill in the art, numerous operational problems may be encountered before, during and after the well intervention work. Hence, operators may find it necessary to rig down certain equipment. In the past, and upon experiencing operational problems, equipment such as the coiled tubing injector head would have to be rigged down so that the encountered problem could be solved. However, due to the various movements being experienced by the floating platform, alignment of the equipment is difficult. A significant amount of time must be spent in rigging up equipment, which equates to significant sums of money due to the day rates of the rig.
Thus, there is a need for a system and method that will allow an operator to move components of a well intervention string assembly with greater ease and simplicity. There is also a need to compensate for motion of a floating platform while undergoing well intervention procedures. These needs, and many other needs, will be met according to the teachings of the present invention.